Transcutaneous electrical nerve stimulation (TENS) devices provide pain relief by electrically stimulating sensory nerves. In general, TENS devices comprise electrodes applied to the skin of a user, and leads or wires connecting each electrode to a main unit. The main unit comprises stimulation circuitry (sometimes referred to herein as the stimulator) and a user interface. The electrodes are placed on the skin of the user within, adjacent to, or proximal to, the area of pain. For conventional TENS devices, the main unit (housing the stimulator and user interface) is typically in a form of a handheld unit that is physically separated from the electrodes. Lead wires, which may be as long as 3 feet, typically connect the electrodes and the handheld unit. Users interact with the TENS device by pressing buttons on the handheld unit and gain feedback from a visual display on the handheld unit.
In users with chronic pain, there is often a need for the TENS device to be worn near continuously with minimal interference to normal daily activities. In this situation, conventional TENS designs (with their long lead wires) can be too cumbersome and prone to accidental detachment of the lead wires from the handheld unit or from the electrodes, or to accidental detachment of the electrodes from the skin of the user. For example, in users with painful diabetic neuropathy, pain is felt primarily in the feet and lower legs, and long lead wires (e.g., extending from the waist to electrodes applied to the upper calf of the user) are especially prone to detachment when users are engaged in normal daily activities such as walking or climbing stairs.
Neurometrix, Inc. of Waltham, Mass. recently developed a TENS device which provides a compact one-piece design that wraps around the upper calf of the user much like a wristwatch wraps around the wrist of a user. Details of the Neurometrix TENS device are disclosed in pending prior U.S. patent application Ser. No. 13/678,221, filed Nov. 15, 2012 by Neurometrix, Inc. and Shai N. Gozani et al. for APPARATUS AND METHOD FOR RELIEVING PAIN USING TRANSCUTANEOUS ELECTRICAL NERVE STIMULATION; pending prior U.S. patent application Ser. No. 14/230,648, filed Mar. 31, 2014 by Neurometrix, Inc. and Shai Gozani et al. for DETECTING CUTANEOUS ELECTRODE PEELING USING ELECTRODE-SKIN IMPEDANCE; and pending prior U.S. patent application Ser. No. 14/253,628, filed Apr. 15, 2014 by Neurometrix, Inc. and Shai Gozani et al. for TRANSCUTANEOUS ELECTRICAL NERVE STIMULATOR WITH AUTOMATIC DETECTION OF USER SLEEP-WAKE STATE, which patent applications are hereby incorporated herein by reference. The low-profile design of the Neurometrix TENS device allows the TENS device be discreetly worn under clothes, and eliminates the aforementioned problems associated with long lead wires extending between the electrodes and the stimulator.
Inasmuch as the aforementioned Neurometrix TENS device is configured to be worn under clothing, it would be advantageous to provide means (in addition to traditional input means such as push buttons) for a user to interact with the TENS device through clothing, e.g., through gestures such as tapping, slapping, or shaking of the TENS device.
In addition to the foregoing, an important safety consideration for TENS devices is the integrity of the electrode-skin contact interface. The TENS electrodes typically utilize hydrogels to create a stable low-impedance electrode-skin interface so as to facilitate the delivery of electrical current to the user, whereby to stimulate peripheral sensory nerves and thereby suppress pain. If the portion of the electrode in contact with the skin decreases (i.e., due to “electrode peeling”, wherein the electrode peels away from the skin of the user), the current density and power density will increase due to decreased electrode-skin contact area. Increased current density and power density can lead to painful TENS stimulation and, in the extreme, thermal burns. Therefore, it is also desirable to monitor the integrity of the electrode-skin interface to safeguard the user's comfort and safety.
While the electrode-skin contact area cannot be easily measured in real-time, the contact area directly affects the impedance to the stimulation current flow, i.e., a reduced electrode-skin contact area will lead to higher impedance. Thus it is desirable to monitor the impedance across the electrode-skin interface in real-time by the TENS stimulator so as to detect electrode peeling. More particularly, if the impedance exceeds a threshold, stimulation should be halted to avoid painful stimulation sensation or potential thermal burns.
Furthermore, electric circuit theory imposes a physical limit to the maximum current a stimulator can deliver to the user, based on the maximum voltage range of the stimulator:[Maximum Current]=[Maximum Voltage]/[Impedance].If the electrode-skin impedance becomes too high, the maximum current deliverable by the TENS stimulator may be lower than the desired therapeutic current intensity. To ensure therapeutic efficacy of the TENS device, the TENS device should measure and monitor the actual current delivered to the user in real-time. If the measured current intensity differs from the target current intensity, then stimulation should be halted.
Wearable TENS devices provide users with pain-relieving therapy while allowing those users freedom to engage in their normal daily activities. However, activities like walking, running, and stair climbing may impose challenges to the accurate detection of intended user gestures and to maintaining a consistent electrode-skin interface. More particularly, mechanical shocks associated with walking or bumping into objects may be detected and incorrectly interpreted as user-initiated control gestures. In addition, ordinary body movements can cause momentary changes in the integrity of the electrode-skin contact and may be incorrectly interpreted as problematic and permanent changes in electrode-skin contact, even though such momentary changes in the integrity of the electrode-skin contact pose no real risk to the patient. Such “false” detections of user gestures, and/or such “false” detection of electrode peeling, can unnecessarily diminish the value of a user gesture detector and/or an electrode peeling detector for a TENS device. For these reasons, it would be advantageous to provide automated means for eliminating “false” detections of user gestures and/or for eliminating “false” detection of electrode peeling in order to increase the accuracy of user gesture recognition and electrode-skin contact detection.